FIGURE 1.

Layered hematopoiesis. The primitive and transient-definitive waves in the yolk sac (YS) give rise to erythro-myeloid progenitors (EMP) that will colonize the fetal liver (FL) consecutively. EMP arising at early stages will differentiate into macrophages and probably mast cells in tissues developing early during embryogenesis (brain, liver), while EMPs at later stages remain in the FL and will give rise to the same cell types driven by the demand of later developing tissues, such as the lung and skin. Hematopoietic stem cells (HSC) develop in the aorta-gonad-mesonephros (AGM) region before colonizing the FL. After the formation of the bone marrow (BM), HSC migrate to the BM cavity where they will constantly give rise to short-lived macrophages and mast cells. The cell of origin for mast cells and macrophages is color-indicated. For simplicity, blood-circulating intermediate precursor stages between EMP and macrophages are not depicted. Created with BioRender.com.